The existing all kinds of digital equipments have a tendency towards higher and higher operation speeds and smaller and smaller sizes for achieving the purposes of saving energy and protecting environment. There are large numbers of DC/DC switching power supplies, which have wide applications, so the required energy and the amount of the power stations can be reduced after performing the energy saving. As a result, the environmental pollution caused by waste water and waste gas exhausted from the power stations can be decreased.
To meet ever-increasing demand for high speed and miniaturization of digital devices, microelectronic circuits are using lower and lower voltage. 5 V and 12 V are no longer dominant power supplies used in microelectronic circuits. 3.3V, 2.5V, 2V, 1.8V, 1.5V, and even 1.2V are becoming standard voltage in many electronic devices. Actually, some next-generation high-speed microprocessors and digital signal processors need 1.0 V as their supply voltage.
Migration to lower supply voltage and size miniaturization is rapidly changing power supply design and packaging technologies. The high switching frequencies together with soft switching and the synchronous rectification technologies help to reduce the losses and size of the power supplies dramatically, thereby further increasing the transformation efficiency.
On the other hand, as the power semiconductors and signal semiconductor devices are getting smaller and smaller, the size reduction of the power magnetic devices, which play critical roles in power supplies, becomes more and more crucial. The use of planar magnetic devices helps to minimize the profile or height of the power supplies. However, the reduction of the sizes of the power transformer and the inductor is the biggest difficulty.
In comparison with the conventional transformer that adopts copper wires as winding coils, the winding coils of the planar transformer is constructed of double-layer or multi-layer printed circuit board or pre-molded planar copper plate. In addition, the planar transformer can be realized upon the successful development of the planar magnetic cores. The planar transformer has significantly increased power density and significantly decreased volume. Accordingly, the volume and thickness of the planar transformer are reduced respectively to only 20 percent and 40 percent of that of the conventional transformer.
The conventional transformer is formed by winding the circular copper wires on the ferrite magnetic core to form winding coil. Therefore, the copper wires can not be fully utilized because of the generation of skin effect, which is especially apparent in high-frequency condition.
When high-frequency electric current flows through a conductor, the change of electric current causes the magnetic field inside and outside the conductor to be changed. According to the electromagnetic induction law, a high-frequency magnetic field creates an induced electromotive force in the conductor on two planes along its longitudinal direction. This induced electromotive force generates an eddy current in the conductor along its longitudinal direction to prevent the magnetic flux from change. The current density of the main electric current and the eddy current is a maximum at the outer edge of the conductor and decreases exponentially towards the center of the conductor. This phenomenon is known as the skin effect. In such a condition, the current-carry area is smaller than the entire conductor area, causing the AC impedance to be larger than DC impedance.
In the planar transformer, the winding is a flat conductor formed by plating copper on the printed circuit board or using the copper plate directly. Although the electric current is focused on the outer surface layer due to the skin effect, the electric current still flow through the entire flat conducting wire for the planar transformer. In comparison with the cylindrical conducting wire, the planar transformer has higher transformation efficiency and power density.
There are examples of “open frame” power converters that rely upon a single mother board technique to create the complete converter including two or more magnetic devices. These magnetic devices that have this configuration are called as embedded planar transformers. Examples include C&D WPA series and Synqor PowerQor series. In these converters, a single multilayer printed circuit board forms the “main circuit board”, which contains primary and secondary windings for transformer. However, this technique requires a large, expensive multilayer printed circuit board such as larger than twelve layers. The heat generated in the multilayer power windings is delivered to temperature sensitive control circuit components, causing the wrong action. Also, magnetic properties are difficult to test; the magnetic device is an integral part of the converter product. Defects in the printed circuit board windings can result in expensive scrap of the entire converter. Any changes on the transformer turns ratio due to the output voltage requirement require the multi-layer printed circuit board to be modified, which results in high cost and high printed circuit board inventory for same platform power supplies with different output voltages.